An object of the invention is the provision of a transmission circuit with a low transmission level and extremely low internal resistance for direct current data transmission over directly connected lines.
It is known in the art of teleprinting to transmit the individual telegraphy signals over the line in the form of direct current pulses with a high level. Such signals, however, are subject to a large amount of crosstalk and distortion. These accordingly occur because of the capacitive and inductive coupling which exists between the individual conductors of a cable, and the cross talk distrubance in the conductors adjacent to the telegraphy line.
A known circuit arrangement, which is to be found in U.S. Pat. No. 3,413,413, shows a transmission system for the transmission of direct current telegraphy signals in the form of voltage level steps. This known method reduces the crosstalk and distortion of the transmitted telegraphy steps in transmissions over symetrically and electrically switched lines. This transmission system makes it possible to transmit the telegraphy signal at the desired speed of transmission with a very low transmitting voltage. The known transmission system uses a so called electronic telegraph relay at the transmitter for the formation of the transmitting voltage. Of necessity, the electronic relay must have a small source impendance. As a result of this small source impedance, only small voltage disturbances and slight distortions of the telegraphy steps or signals occur. The purpose of this telegraph relay is to receive the telegrapy signals from the data input apparatus, such as a teletypewriter, to regenerate that signal, i.e., correct distortions, and to then pass the signal on with the same polarities, but with the described small transmitting voltage. The reconstituted telegraph signal is coupled to an output circuit and from there to a transmission line. The known electronic telegraph relays are electrically isolated, and for this reason, they really function so that an oscillator is formed which gives off an alternating voltage. Each telegraph step can then assume one of two conditions, i.e., character current condition or separation current condition. One of the two conditions of the telegraph step actuates the oscillator so that it oscillates and generates an alternating voltage, while the other condition blocks the oscillator so that no alternating voltage can occur. The alternating voltage is inductively decoupled and rectified. The rectified voltages are applied to two parallel connected diodes, which, however, are connected for current flow in directions of opposite polarity. A resistance is connected in series with the diodes. Using a voltage divider, the voltage at the diodes is set to the value of the transmission voltage, which, for example, may be plus or minus 0.4 volts. The attainable internal resistances thereby are about 10 Ohm. The temperature dependence of this circuit is very high because of the temperature coefficient of the diodes, so that the influence of the temperature must be reduced with temperature-dependent resistors in the voltage divider. However the setting of these low valued resistors is very difficult. The known telegraph relays have large current requirements since in the limit a large current flows over the resistances and the diodes. An alternating voltage oscillator is used to supply this current requirement; the oscillator operates at a sufficiently high frequency that distortions of the telegraph signal are as small as possible. The oscillator emits a relatively high valued current so that it is difficult to suppress interference from it or to prevent it from forming disturbances which occur in the frequency ranges of wireless transmission sources or the like.